The greatest assurance over management's assertions would be provided by evaluating the completeness of the report and management's responses to identified deficiencies. This involves ensuring that all relevant control processes and deficiencies have been identified, and that management has provided appropriate and comprehensive responses to each issue. This step ensures that the internal control assessment is thorough and that management is actively addressing any weaknesses. Reference:
The IIA's International Standards for the Professional Practice of Internal Auditing (Standards), specifically Standard 2130 - Control.
The IIA's Practice Guide on Assessing the Adequacy of Internal Controls.